Natural gas vehicles (NGVs) are alternative fuel vehicles that run on compressed or liquefied natural gas, which is a cleaner energy source. Compressed natural gas (CNG) is made by squashing the gas to less than one per cent of the volume it occupies at standard atmospheric pressure.
It’s stored in highly pressurised containers (usually metal canisters) at a pressure of between 176-253 kilograms per square centimetre (2,900-3,600 pounds per square inch), from where it’s distributed to the engine. The canisters are typically installed in the boot or under the rear seating.
In contrast, liquefied natural gas (LNG) is made by turning natural gas into a liquid state via a process that involves removing any condensates, such as water and oil, before cooling the gas. The cooling process culminates in a purified liquid form of natural gas, which is approximately twice as dense as CNG.
The LNG can then be stored in relatively low-pressure canisters (about 4.9-10.5 kilograms per square centimetre/70-150 pounds per square inch) and integrated into a vehicle in a similar way to CNG varieties. Importantly, as the LNG is liquefied, prior to combustion in the engine, it must first be re-converted into gas via a vaporiser.
Overall, NGVs are becoming more common on the market as both CNG and LNG systems deliver ‘greener’ emissions compared to standard petrol and diesel vehicles.